Your water looks a bit cloudy. It could be that you didn't rinse the sand off enough. Don't get me wrong your tank is looking good. What filter are you using. As you are adding more fish you may have to upgrade your filtration. My tank is 4f 240l, it's fully stocked now. My fish are between 2 inces and 7 inches . I have 3 juvies and 22 adults. I'm a newbie to this just like you. I've only been keeping mbuna for 8 months. I started with a tetratech ex1200 and now just added an eheim 2215 classic. As you add more fish and they grow your bio load will increase. Other than nitrate levels (40ppm) my water has been great. The extra filter and the shed load of matrix should help. And just one more thing the argos play sand can be a pain in the arse. It's very fine and light and can end up in your filter. Don't worry I've made loads if mistakes. All I can say is that you learn by your mistakes and move on. Wellcome to the joys of keeping Malawi.